1. Field of the Invention
The present invention relates to electromagnetic couplings, and in particular, a contactless rotary coupler.
The objective is to suggest a design of a rotary coupler working at UHF, in particular in the 400-500 MHz frequency range, that provides a contactless link between one or two resonant sensors installed on the two opposite sides of the rotating shaft and the stationary electronic interrogation unit. The coupler should ensure                (a) a maximum amplitude of the resonant sensor response seen at the stator input of the coupler,        (b) a minimum variation of the response amplitude and        (c) a minimum variation of the resonant frequencies of the sensors with the rotation angle.        
2. Description of the Related Art
1. Racal patent WO 96/37921 (hereinafter, Racal)
The patent discloses a rotary coupler that is based on a quarter-wave coupled-line directional coupler (see FIG. 1a), a well-known four-port microwave device. The difference between it and the proposed coupler is that the coupled transmission lines of the latter are not linear but annular (FIG. 1b) with the circumference close to λ/4 (or 0.62λ/4 to minimize phase and amplitude variation of S41 with the rotation angle).
In order to achieve a total power transfer from port 1 of the stator ring 10 to port 4 of the rotor ring 20, the quarter-wave 3 dB coupler can be loaded as shown in FIG. 2a. Ports 2 and 3 are short-circuited and the output port 4 is loaded by Z=Z0 where Z0 is the characteristic impedance of the external circuit. It is important to note that the load Z is always connected between the end of the strip and the ground plane (it is not shown in the figures) because the transmission line ports are defined this way.
The rotary RF or microwave coupler is needed for a torque sensor based on Surface Acoustic Wave (SAW), STW and FBAR resonators or other types of resonant structures sensitive to strain on the shaft surface. It can also be used to do temperature measurements and other types of measurements on rotating shafts. We are interested only in the sensor application of the rotary coupler although it is widely used in other areas (e.g. radars). Further on we shall use the term SAW sensor to denote any type of the resonant structure sensitive to physical quantities of interest. The aim of the interrogation unit is to measure die resonant frequency of the SAW sensor. If the sensor 30 is connected to the rotor ring instead of the load Z as shown in FIG. 2b then the interrogator can easily “see” the resonant peak in S11, the frequency response at the stator port 1, and do the frequency measurement.
For the sensor application, it is not essential to have a strictly defined amount of coupling between the stator and the rotor rings (3 dB coupling, for instance) and a strictly defined circumference length of the coupler (λ/4 for instance) in order to be able to measure the resonant frequency at port 1. The resonant peak in S11 exists within a wide range of the coupler geometrical parameters but its amplitude and position depend to a large extent on the geometry of the coupler disclosed in the Racal patent. For some shaft diameters and frequencies it is quite difficult to obtain a well-pronounced resonant peak at any rotation angle.
For sensor applications two aspects are important:                (a) the amplitude of the resonant peak in S11 should be as large as possible and        (b) the variation of the amplitude and the position of the resonant peak in S11 with the rotation angle should be as small as possible.        
Transense patents quoted below are devoted to the solution of this problem.
2. Transense patent application GB 2328086 (hereinafter, Transense '086)
This application differs from the Racal patent by the addition of the trimming capacitor between the terminals 1 and 2 of the stator ring in order to slightly broaden the coupler bandwidth and reduce the angular variation of the resonant frequency seen at port 1. The SAW sensor is connected between the terminal 4 of the rotor ring and the ground as shown in FIG. 2b. If the sensor contains more than one SAW resonator then each of them should be connected to a separate rotor ring coupled to a separate stator ring. According to this application, all the stator and rotor rings can be on the same stator and rotor boards. However, being concentric they will have different diameters and as a result the resonant peaks seen at the stator inputs will vary differently with the rotation angle. As a consequence measuring the difference between the resonant frequencies will not allow efficient cancelling of the angular frequency variation.
3. Transense patent application GB 2368470 (hereinafter, Transense '470)
This application discloses a coupler similar to that described in the previous patent application. In fact it consists of two Racal-type couplers each forming not a full circle but just half a circle and connected in parallel. This allows using the coupler with the shafts of a larger diameter so that the total coupler circumference is larger than λ/4. The SAW sensor is again connected between the stripline end and the ground plane.
4. Transense patent application 2371414 (hereinafter, Transense '414)
The coupler disclosed in this application is not based on electro-magnetically coupled transmission lines as it was in all previous patents. It utilises two purely magnetically coupled loops with the grounded electric screen between them that prevents a coupling by means of electric field. This coupler should work all right at low frequencies where the circumference is considerably shorter than the wavelength. At higher frequencies, due to the absence of ground planes on both sides of the coupler and poor field confinement, there will be considerable radiation losses and the coupler will also be susceptible to interference. Small signal amplitude at the input of the stator can also be problematic for this coupler.
5. Paper by O. Shteinberg and S. Zhgoon (hereinafter, Shteinberg)
The paper describes the coupler consisting of two annular coupled transmission lines as shown in FIG. 2c The SAW resonator connected between the terminals 3 and 4 instead of being connected between terminal 4 and ground as it is in Transense '470.